This invention relates generally to DC power supplies and is particularly directed to a transformerless power supply capable of providing a plurality of well-regulated DC output voltages.
DC power supplies energized by an AC input typically employ a diode rectifying bridge for converting the alternating input to a DC output voltage. This type of power supply generally utilizes a non-polar capacitor coupling the AC source to one input of the rectifier for reducing the input voltage to a more convenient level. A non-polarized capacitor is utilized because of the reverse voltage applied across it in the AC line. A capacitor of this general type utilizes electrolytic material capable of supporting high currents and voltages which adds to the expense of the DC power supply. In some DC power supplies of this general type a resistor is utilized in place of the aforementioned non-polar capacitor. However, the use of a resistor generally results in excessive power dissipation in the power supply.
Thus, prior art DC power supplies incorporating a rectifying bridge are basically capacitive or resistive networks converting a high input voltage to a low output voltage and, as such, operate as a constant current source. Such power supplies generally permit the output voltage to assume any value (within reason) necessary for providing a constant output current. This is undesirable in that excessive currents and voltages may be produced by such power supplies. In addition, a series regulator is required for output voltage regulation. The series regulator tends to use more power than a voltage regulator in parallel with the power supply and to impose a varying load on the power supply making output voltage regulation very difficult. As such, these series regulated power supplies are generally not capable of providing a plurality of well-regulated DC output voltages without incorporating a transformer in the power supply which necessarily increases the cost and the complexity of the power supply.
U.S. Pat. No. 4,198,660 to Ferracini discloses a DC supply circuit wherein a resistor is coupled to one input of a full wave bridge rectifier for limiting the current provided thereto. As previously stated, this approach to input voltage reduction to a more manageable level by current limiting results in excessive power dissipation in the power supply. U.S. Pat. No. 4,207,516 to Babcock discloses a switching regulator for regulating an input voltage having an AC component which employs a full wave bridge rectifier at the input thereof. The full wave rectifier is conventional in design, incorporating four coupled diodes. Across each diode is shunted a bypass capacitor and across the entire rectifier bridge is coupled an RF bypass capacitor. The former bypass capacitors are subjected to the alternating voltage applied across the respective diodes and thus are non-polar capacitors, while the latter bypass capacitor performs a signal filtering function with respect to the output of the rectifier bridge. This power supply includes of series switching regulator and, as such, is not capable of providing a plurality of well-regulated DC output voltages.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a DC power supply capable of generating a plurality of well-regulated output voltages from an AC input without utilizing a transformer and in which power supply cost is reduced by utilizing inexpensive, commonly available polar capacitors.